Anycast is a network addressing and routing methodology whereby the same network addressing is advertised from two or more different network locations. FIG. 1 conceptually illustrates an Anycast based network architecture. There are two different network sites, also referred to as points-of-presence (“PoPs”) 110 and 120, from which one or more servers provide different requesting clients with access to the same and/or different content, services, and/or data.
Each PoP 110 and 120 advertises that it is accessible at the same Internet Protocol (“IP”) address. In particular, the gateway router at each PoP 110 and 120 distributes Border Gateway Protocol (“BGP”) advertisement messages (i.e., BGP update messages) to neighboring network routers. The advertisements convey the number of hops to an address through a sequence of Autonomous System (“AS”) identifiers and/or other identifiers. The advertisement messages identify the same IP addressing for reaching each PoP 110 and 120.
The other network routers receive the advertisements and build routing or forwarding tables that identify the shortest path to each advertised address. Thus, when a request directed to the Anycast address advertised from PoPs 110 and 120 is received by router 130, router 130 identifies PoP 110 as the closest destination, and routes the request to PoP 110. When a request directed to the Anycast address advertised from PoPs 110 and 120 is received by router 140, router 140 identifies PoP 120 as the closest destination, and routes the request to PoP 120 instead of PoP 110.
Anycast involves relinquishing control of the routing to the network and/or routers along the network paths to PoPs 110 and 120. As a result, Anycast does not allow for deterministic shifting of a specific subset or a specific amount of traffic from one Anycast site to other specifically selected Anycast sites. For instance, pulling advertisements of one or more Anycast addresses from the gateway router of a site is one manner to shift traffic away from that site. However, the amount of traffic, load, and/or clients that are shifted from the site, and the one or more destination sites that receive the shifted traffic, load, and/or clients are not deterministically controllable.